The advent of directional drilling, and of logging while drilling (“LWD”) and measurement while drilling (“MWD”), have profoundly improved the locating and placement of wells for production of hydrocarbon resources. Directional drilling involves the drilling of a well bore along a deviated course for various purposes, including bottoming out in a particular target region. Directional drilling is employed, for example, to obtain an appropriate well bore trajectory into an oil producing formation bed (or “pay zone”) and then drill substantially within the pay zone. A horizontally drilled well can greatly increase the borehole volume in the pay zone with attendant increase in oil production.
LWD usually refers to measurement of formation properties with instruments that are located in the drill collars immediately above the drill bit. Measurements are made shortly after the hole is cut and before it is adversely affected by continued drilling or coring operations. Fluid invasion into the borehole wall is also reduced relative to wireline logging because of the shorter time between drilling and measurement. LWD and MWD are sometimes distinguished in that whereas LWD data are recorded in memory and downloaded when the tools reach the surface, MWD data are transmitted to the surface (e.g. by mud pulse or wired drill pipe) and monitored in real time. However, the term LWD is also used more generically to cover both LWD and MWD, and will be used in that manner herein, unless otherwise indicated.
Before a directional drilling well plan is devised, the drilling team will often have significant a priori knowledge of geological attributes of the local formations. Initial knowledge may be derived, for example, from survey and/or producing wells in the local area. Seismic surveys are commonly used in locating the well path in the formation, due to its ability to probe far into the formation. However, seismic surveys are generally incapable of providing the desired resolution and accuracy to sufficiently characterize a target formation and predict location for advantageous well placement.
Other measurements, such as resistivity measurements, are often made with electromagnetic (EM) tools. Some EM logging tools use propagation techniques to measure the resistivity of the formation. A propagation tool measures the amplitudes, phase shifts, and attenuation of EM signals in the formation to determine the resistivity of the formation, which can be an important aspect of its characterization.
A resistivity tool that provides a relatively deep reading is disclosed in U.S. Pat. No. 6,188,222 assigned to the assignee of the present invention. This tool, which can be part of an LWD tool array, includes a relatively long spacing between the transmitter and the receiver, and can be used to determine formation resistivity and to obtain indications of formation boundaries for determining well location during directional drilling. Subsequent developments, in LWD tools, some of which are summarized hereinbelow, have improved the accuracy of controlled directional drilling and have facilitated well placement. However, there is still much room for improvement of these functions, and of the characterization of target earth formations and their reservoirs, and it is among the objectives hereof to achieve such improvement.